One day in 2005, neuroscientist James Fallon was casually leafing through the PET scans of serial killers. As one does. He was doing research at UC Irvine and was trying to figure out which aspects of brain anatomy contribute to psychopathy.

All children play up from time to time, but deep-down, some are incredibly badly behaved. While it's easy enough to spot them in the playground, researchers are struggling to come to terms with whether it's possible to diagnose children as being real—and perhaps even dangerous—psychopaths.